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In the context of the current healthcare reform debate, the ongoing, rapid growth of cardiac imaging has triggered efforts to improve appropriate use and contain costs of diagnostic testing. As part of this, the process of test evaluation and validation has shifted from a focus on diagnostic accuracy to the use of risk-based criteria. Optimally, evidence of improved risk stratification directing subsequent treatment decisions will justify and guide appropriate use of an imaging test, in comparison with other diagnostic strategies. Such comparative effectiveness data is still sparse for cardiovascular imaging, and current clinical guidelines are often based on expert consensus rather than evidence alone (1).

Coronary imaging is a representative example. In the late 1950s, the introduction of coronary angiography completely transformed the treatment of patients with coronary artery disease (CAD). Even though large outcome trials satisfying current standards were lacking, coronary angiography was rapidly integrated into clinical care, and for a long period, was considered the “Holy Grail” in cardiology. The common strategy of imaging was the identification of focal, hemodynamically significant coronary lesions and associated ischemia justifying coronary revascularization. In recent years, there has been a shift from the identification of hemodynamic stenosis alone to image-based identification of patient risk. Clinical trials using such data as the basis for subsequent treatment decisions have allowed better definition of the benefit of revascularization versus optimal medical management (2).

Secondary analysis of data from imaging studies performed for the identification of hemodynamically significant coronary stenosis has demonstrated that these tests yield additional prognostic value for the prediction of future cardiovascular events. For functional modalities (echocardiography, cardiac magnetic resonance, perfusion imaging), left ventricular ejection fraction, extent of ischemia, and scar are strongly associated with adverse outcomes. For anatomic modalities, the extent and distribution of atherosclerotic disease burden (combining luminal stenosis and atherosclerotic plaque burden, plaque morphology, and composition) have prognostic implications.

This concept has been well validated with coronary computed tomography angiography (CTA), with multiple studies demonstrating that the extent and severity of coronary atherosclerosis has prognostic value (3–6). A recent meta-analysis summarized results from 18 studies including 9,592 patients with suspected or known CAD (7). During a median follow-up of 20 months, 449 major adverse cardiovascular events (MACE) occurred, including 180 deaths, 56 myocardial infarctions (MI), and 213 revascularizations, consistent with an event rate of 4.7% for MACE and 2.4% for death and MI. The pooled annualized event rate for obstructive (any vessel with >50% luminal stenosis) versus normal coronary CTA was 8.8% versus 0.17% per year for all MACE (p < 0.05) and 3.2% versus 0.15% for death or MI (p < 0.05). Strata of “absent CAD,” “nonobstructive CAD” (worst stenosis <50%), or “obstructive CAD” demonstrated incremental increases in the risk of future MACE (0.17%, 1.41%, and 8.84%, respectively). The data demonstrated several important issues. First, adverse cardiovascular events among patients with normal findings on coronary CTA were rare, similar to the background event rate among healthy low-risk individuals (<1%). Interestingly, this event rate was lower than that previously reported from other imaging modalities including stress echocardiography and myocardial perfusion imaging (8–10). Nonetheless, coronary CTA satisfied the basic tenets of successful risk stratification (i.e., identification of a low-risk patient subset, defining gradations of risk on the basis of test abnormality and concentrating risk in patients with abnormal tests) (11). Second, the presence of “significant” stenosis (>50% luminal diameter stenosis) is associated with significantly higher risk than nonobstructive atherosclerotic changes, supporting observations that have in the past been made with invasive coronary angiography. Lastly, although adverse events were significantly more common among patients with abnormal coronary CTA findings, the majority of patients with disease did not have adverse outcomes. In other words, even a positive computed tomography (CT) scan was not strongly predictive of future MACE.

The paper by Schlett et al. (12) in this issue of iJACC provides further data concerning the prognostic value of coronary CTA. It focuses on a specific patient group; patients who had presented to the emergency department with acute chest pain but negative initial troponin and electrocardiogram. It is an extension of the ROMICAT (Rule Out Myocardial Infarction/Ischemia Using Computer Assisted Tomography) trial in which the use of coronary CTA for the initial management of such patients had been investigated (13). Schlett et al. (12) now report the 2-year follow-up of 333 patients. The presence of plaque, luminal stenosis (>50%), and left ventricular regional wall motion abnormalities (RWMA) were assessed. The primary endpoint was a composite of cardiac death, nonfatal MI, and coronary revascularization. At the end of the follow-up period, 25 patients had 35 MACE (0 cardiac deaths, 12 MI, and 23 revascularizations), for an event rate of 6.8% for MACE and 3.3% for death or MI. The cumulative probability of 2-year MACE increased across CT strata for CAD (no CAD 0.0%, nonobstructive CAD 4.6%, obstructive CAD 30.3%; log-rank p < 0.0001) and across combined CT strata for CAD and RWMA, thus achieving significant risk stratification. The C statistic for predicting MACE was 0.61 for clinical Thrombolysis In Myocardial Infarction risk score, improved to 0.84 by adding CT CAD data, and further improved to 0.91 by adding RWMA (both p < 0.0001).

Several important issues merit mention. First, the authors did not differentiate between revascularization performed early versus late after coronary CTA (before vs. after 60 to 90 days post-testing). The latter is an accepted endpoint related to progression of CAD, whereas the former is more critical. It is related to the results of initial presentation and workup and thus can be a very different phenomenon. Previous studies that addressed the use of coronary CTA for diagnostic purposes typically excluded early revascularizations (6,14,15) to avoid a “self-fulfilling prophecy”: a positive CT triggers revascularization, which in turn would be used as evidence that CT has prognostic relevance. However, in the ROMICAT trial, the results of CTA were not communicated to the treating physicians and hence—different than other prognostic trials of coronary CTA—CT interpretation did not influence decisions on early revascularization. This mitigates some of the problems of including early revascularizations. As another relevant issue, the study cohort had an overall hard event rate (cardiac death and MI) of 1.9% per year and therefore constituted a relatively low-risk group (12 MI in 333 patients over 23 months). Hence, 333 patients were tested to identify 183 at low risk (no obstructive CAD) and 25 who went on to have events, most of which were revascularizations, a relatively poor yield. Finally, it was unclear whether the results of coronary CTA in these patients could have resulted in a change in patient management that would have yielded improved patient outcomes, an important criterion for imaging in a value- or benefit-centered healthcare setting.

Interestingly, this issue of iJACC contains another article on the prognostic value of coronary CTA (16), in patients with prior coronary bypass surgery, a patient group at substantially higher risk. Of note, similar prognostic data have also been described in recent CT studies examining calcification and plaque of the aorta (17–19). Potentially, review of CT scans performed for a wide range of appropriate clinical indications (e.g., chest CT for pulmonary embolism, assessment of the aorta, pre-operative planning) could provide such prognostic data.

These results are in line with our understanding of CAD (20). Atherosclerosis is a systemic, inflammatory process occurring in the entire arterial tree with various patterns of distribution and rates of progression. Individual “vulnerable” lesions undergo clinically unpredictable, sudden changes (rupture, erosion, thrombosis), which in some, but not all, instances initiate clinical events, including myocardial or cerebral ischemia/infarction and limb ischemia/necrosis. Current diagnostic modalities cannot reliably identify specific markers of impending acute events (plaque vulnerability) and therefore do not allow precise prediction of individual events. However, post-test risk is greater in patients with clinical cardiovascular risk factors, biochemical risk markers (inflammation), and higher atherosclerotic disease burden. Because of the systemic nature of atherosclerosis, it is plausible that evidence and extent of atherosclerotic disease (stenosis and nonobstructive plaque) in any vascular arterial bed indicates higher risk of future cardiovascular events.

Although convincing and in line with our understanding of the underlying systemic atherosclerotic disease process, these data are still incomplete (21). Understanding the relationship between diagnostic imaging and outcome is further complicated by the indirect relationship between the former and the latter. Imaging impacts subsequent patient outcomes only through its influence on downstream physician decision making. The relationship between imaging and downstream testing and therapeutic interventions is not well understood, and many studies of post-imaging outcomes fail to control or risk-adjust for post-imaging patient management. This is a particular concern in the setting of data collection in randomized clinical trials examining noninvasive testing.

Further, applying a risk estimate to individual patients requires a multicomponent, validated score, a much-needed tool missing from our current imaging armamentarium. Finally, as is the case for most imaging prognostic data, it is important to incorporate risk assessment of patients treated both medically and with revascularization, thus permitting estimates of relative benefit and identification of optimal patient management.

In summary, with the shift to a value-based healthcare system, the value of a test for the evaluation of patients with known or suspected CAD is increasingly seen in the ability of the test to risk-stratify and direct downstream management, leading to improved clinical outcomes. Although data for individual modalities are accumulating, the relative value of different modalities in specific clinical scenarios has not been systematically compared. Such an assessment will also need to weigh risks of the test, including potential downstream consequences of false positive results, contrast administration, and radiation exposure (22). For example, when compared with conventional angiography (7 mSv), sestamibi myocardial perfusion imaging (12 mSv), and recent low-dose coronary CTA acquisition protocols (3 mSv) (23,24), the radiation exposure in the article by Schlett et al. (12) is relatively high (estimated dose 14.7 ± 2.2 mSv).

What we truly need are large, prospective clinical trials that connect diagnostic testing with clinical endpoints. One of the central questions in study design will be whether downstream treatment decisions following the imaging test will be mandated. Such trials are currently enrolling patients, some mandating treatment options (25) and others not (26). Over the next several years, these data will define the relative role of invasive and noninvasive modalities and their cost effectiveness (27).

We currently face a situation with tremendous opportunities but also tremendous challenges and responsibilities: On one hand, imaging tests have matured—such as coronary CTA, which continues to provide ever more accurate images at ever lower radiation exposure. Just as importantly, analyses such as the studies by Schlett et al. (12) and Chow et al. (16) in this issue of iJACC demonstrate prognostic value of this new test in specific patient populations. As outlined earlier, ongoing larger trials using CT and other imaging modalities will provide even more information regarding prognostic implications of noninvasive imaging. Hence, the medical community is gaining new tools of substantial potential value. The tremendous responsibility of researchers and clinicians is to define and adhere to appropriate uses that translate into true benefit for patients and society, improving outcomes while avoiding higher healthcare expenditures. We urgently need large-scale trials so that clinical guidelines based on comparative effectiveness data, rather than expert consensus alone, will direct individual patients to the most appropriate diagnostic approach and further treatment. Tremendous opportunities are in our hands—it is up to us to use them to the maximum benefit of the patients who entrust us with their well-being.

Footnotes

Dr. Achenbach has received research grants from Siemens Healthcare and Bayer Schering Pharma and has served as a consultant and on advisory boards for Servier, Circle, and Guerbet. All other authors have reported that they have no relationships to disclose.

↵⁎ Editorials published in JACC: Cardiovascular Imaging reflect the views of the authors and do not necessarily represent the views of JACC: Cardiovascular Imaging or the American College of Cardiology.

American College of Cardiology Foundation

References

(2010) ACCF/SCCT/ACR/AHA/ASE/ASNC/NASCI/SCAI/SCMR 2010 appropriate use criteria for cardiac computed tomography: a report of the American College of Cardiology Foundation. J Am Coll Cardiol56:1864–1894.